In recent years a need has developed for low power, low cost and light weight display devices. EPDs (Electrophoretic Displays) can fulfil this requirement. One use of EPDs is for electronic paper. It is imperative that once an image is displayed, the image can be retained for a long period of time without further voltage being applied. Hence, this fulfils the requirements of low power use, and means an image can be visible until another image is required.
An EPD generally comprises charged electrophoretic particles dispersed between two substrates, each comprising one or more electrodes. The space between the electrodes is filled with a dispersion medium which may be a different colour to the colour of the particles. The space between the electrodes may also be filled with a transparent dispersion medium and two kinds of particles with charge of opposite signs. If a voltage is applied between the electrodes, charged particles move to the electrode of opposite polarity. The particles can cover the observer's side electrode, so that a colour identical to the colour of the particles is displayed when an image is observed from the observer's side. Any image can be observed using a multiplicity of pixels. Available technologies of EPDs include electronic paper, commercially used in electronic books. This application uses black and white colour. However, the main disadvantage of state of the art EPDs is the lack of a bright full colour system.
The use of different coloured particles in a single pixel has been exemplified in recent patent literature (U.S. Pat. No. 7,304,634, GB 2 438 436, US 2007/0268244), but all of these approaches require the use of complex cell structures and drive schemes. Special coloured particles for EPDs and processes for their preparation are disclosed in US 2007/0297038, US 2008/0013156, U.S. Pat. No. 6,822,782, WO 2007/048721, WO 2008/003619, WO 2008/003604, US 2005/0267263, WO 2006/126120, and J. Nanosci. Nanotechn. 2006, Vol. 6, No. 11, p. 3450-3454. Two particle system comprising inorganic and resin particles are also known (EP 1 491 941). These coloured particles are only achievable by complicated processes and/or they are only suitable for specific applications. Similar coloured particles and their preparation processes are known for analytical techniques (U.S. Pat. No. 5,607,864 and U.S. Pat. No. 5,716,855) and as toner particles for ink jet printing (U.S. Pat. No. 4,613,559).
In EPD, in order to move particles effectively in a non-polar fluid, and to avoid flocculation of particles, the particles require to be sterically stabilised and charged. Reported methods of preparing polymeric particles suitable for EPD are complicated and have numerous steps. Graft copolymer stabilisers made up of poly(12-hydroxystearic acid)-(glycidyl methacrylate)-(methyl methacrylate) are known in prior art (e.g. K E J Barrett, Dispersion Polymerisation in Organic Media, Wiley Interscience 1975). PMMA-b-PODA stabilisers are reported (Harris and Holder. Polymer 47, 2006, 5701-5706). However, many reported stabilisers prepare particles which are too large for EPD or require multiple steps, or use too dilute conditions or require the presence of specific groups to be present in the particles to be captured or can be washed out or removed over time, are not commercially available or are not suitable for dispersion polymerisation in non-aqueous solvent.
There is a need to simplify the complicated preparation of polymeric particles suitable for EPD by means of a simple preparation of charged coloured particles which can be easily dispersed in non-polar media, show electrophoretic mobility and which do not leach colour in a dispersant.